The intestinal mucosa is at equipoise with a complex luminal milieu. The broad goals of the studies planned in this project are the elucidation of the mechanisms of intestinal epithelial interaction with luminal bacteria and the factors defining the functional mucosal response to this interface. The outcome of these interactions determine whether indeed the bacteria is a commensal (or possibly a symbiote) or a pathogen. The importance of this discriminant function is underscored by the recent demonstration of the presence of adhesive invasive E. coli (AIEC) with biologic behavior thal is intermediate between classical commensal and truly invasive pathogens and that are selectively associated with the mucosa of patients with Crohn's Disease. Recent findings suggest that a family of intracellular receptors designated Nods (Nods 1 and 2;also designated Card 4 and 15 respectively) play a critical role in mediating epithelial responses to bacteria which may orchestrate the overall outcome of the bacterial-mucosal interaction. This project tests three broad hypotheses: 1) Nod proteins monitor for the presence of intracellular bacteria 2) That commensalism and symbiosis reflects the nature of the epithelial response to the bacteria as much as the intrinsic invasive properties of the bacteria itself and 3) Variant forms of Nod2 result in aberrant epithelial responses to bacteria that eventuate in chronic inflammatory bowel disease. These hypotheses will be tested in aggregate through studies aimed to address three specific aims: Specific Aim I: To delineate components of the signaling pathways activated by the Nod proteins in intestinal epithelial cells. Specific Aim II: To define the functional responses of intestinal epithelial cells mediated by activation of Nod pathways. Specific Aim II: To define alterations in epithelial functional responses to bacteria mediated by disease associated mutant Nod2 in vitro and in vivo. Collectively, these studies should provide insight into key cellular mechanisms that enable homeostatic co-existence with a normal flora while also enabling the intestinal epithelium to mount a primary response to bacteria when they first breach the integrity of the mucosal barrier.